The present invention relates to actuator mechanisms. More particularly, the present invention relates to systems for providing accurate mechanical control of a movable member in a linear direction.
In a wide variety of applications, it is important to provide precise linear motion of an actuator member in response to a control signal. For example, in many industrial and robotics applications such precise control of linear motion is very important.
For example, one industrial application where such linear control is important is in valve systems for controlling the flow of steam or other fluids which need to be controllably released. In such applications, a piston or some other type of value stem is moved linearly under the control of a linear actuator to open a valve opening by an amount corresponding to the amount of linear motion. Linear actuators commonly employed for such applications include pneumatic and hydraulic driven pistons, worm gears and cam driven rods. Such prior art linear actuators used in control valves have several disadvantages, however. In particular, in applications requiring control of high temperature fluids such as steam, or low temperature fluids such as refrigerants, the temperature variations in the valve system can result in variations in the response of the linear actuator. For example, stiction in the actuator may occur such that when a control signal is applied, the actuator may move in a discontinuous manner or not at all. Such variations are uncontrollable and unpredictable thereby creating inaccuracies in the operation of the valve. The resulting variations in the amount of fluid controlled by the valve may have serious consequences for industrial process control. Additionally, such problems will be important in applications where a relatively short linear stroke for the actuator must be controlled accurately and/or under substantial load, or when substantial variations in operating conditions arise.
Another problem experienced in many applications of the aforementioned linear actuators relates to the inherently analog nature of such actuators. In order to provide more than two linear positions, such actuators require an analog control signal provided from a control system to drive the linear actuator by the desired amount. The control electronics employed in industrial and robotics is becoming predominantly digital in nature, however, frequently being under microprocessor or larger computer control. This requires interface electronics to allow the digital control system to control the inherently analog linear actuator. Alternatively, an on/off approach has been employed, for example, using on/off electrical solenoids, to provide a simple two-way linear actuation system suitable for digital control. However, such a two-way system cannot provide the more detailed adjustments necessary for many applications. Therefore, for example, to obtain a range of fluid flow through a two-way valve a plurality of valves must be employed, each driven on or off by an on/off linear valve actuator, to give a desired range of control. Additionally, many applications other than valve control require more than the simple on/off linear actuation provided by such solenoid type actuators.
Additionally, prior art linear actuators providing continuous linear motion, such as worm gear, pneumatic or hydraulic actuators, generally lack any positive mechanical calibration of the actuation distance. Rather, such actuators depend on the control electronics to ensure the correct amount of linear actuation. Alternatively, complicated and expensive sensors and feedback control systems are needed to ensure the mechanical actuator has correctly responded to the analog control signal to provide the desired amount of linear travel. For many applications requiring very compact systems or operating in extreme or demanding environments, such sensors and feedback control systems can involve extremely difficult design problems and considerable expense. For example, this is especially true in the harsh, corrosive environment of paper making which requires extremely high reliability coupled with continuous production schedules.
Accordingly, a need presently exists for a linear actuator providing precise and positive linear motion compatible with digital control systems and not requiring complex feedback control.